Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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FXUS64 KFWD 211717

Area Forecast Discussion
National Weather Service Fort Worth TX
1217 PM CDT Thu Sep 21 2017

/18z TAFs/

The overnight intrusion of stratus which impacted all sites along
the I-35 corridor continues to lift and thin early this afternoon
as the still potent late-September sun facilitates strong
boundary layer mixing. VFR conditions will prevail through the
rest of the day and evening along with southeasterly winds of
around 15 kts. Some occasional gusts to 20-25 kts will be possible
today. There is some potential for sea breeze activity to push a
couple showers towards the Waco airfield later this afternoon, but
relatively subsident air overhead should help minimize the threat
for impacts from thunder.

An additional round of MVFR cigs is anticipated tonight as flow
in the 925-850 mb layer increases to 25-30 kts and once again
draws additional moisture northward atop the cooling boundary
layer. Forecast soundings show this moist layer may be a bit
shallower than what impacted our sites this morning, and
scattering to VFR should therefore occur a bit more hastily.
Sufficient moisture may linger in the low-levels for a rogue
shower near Waco during the heating of the day, but this potential
is much too low to warrant a mention in the TAF.



.UPDATE... /Issued 1140 AM CDT Thu Sep 21 2017/
A healthy surge of low stratus is a sign that moisture remains in
good supply across the region. This low cloud cover along and
east of I-35 will gradually thin into the afternoon leaving most
of us with mostly sunny to partly cloudy conditions today.
Regional radar imagery already depicts showers blossoming across
the Upper Texas Coast with the addition of a bit of daytime
heating, and anticipate this trend to continue farther northward
this afternoon. While a stray shower or storm cannot be ruled out
as far north and west as a Paris to Terrell to Lampasas line
today, fairly subsident air inferred just under 500 mb on our
morning sounding should really work against more appreciable
coverage. We`ve confined the highest PoPs (30-40%) south and east
of an Athens to Cameron line within a maximized theta-e/PWAT axis.
As stated in the previous discussion, any of the more robust
cells will be capable of producing some strong downburst winds
given high DCAPE values and respectable surface-700 mb theta-e

Farther north and west, a very subtle wave is evident in 400-250
mb streamlines, and some ascent can be inferred from streaks of
developing altocumulus across the Big Country and Rolling Plains.
Most hi-res guidance remains silent here this afternoon, possibly
a result of the somewhat more subsident airmass referenced
earlier. Thus, opted to leave the forecast dry with a silent 10%
PoP for now with indications that any thunderstorm activity should
remain displaced just west of our area across the Rolling Plains,
but will monitor cloud and model trends this afternoon.



.SHORT TERM... /Issued 325 AM CDT Thu Sep 21 2017/
The main forecast concern today will be the extent of convection
associated with some warm advection/seabreeze activity in
southeast Texas. An ongoing surge of moisture will cause PWs to
climb into the 1.7-1.8" range across our southeastern counties
by midday, and weak ascent in a mostly uncapped environment
should lead to the development of scattered showers and a few
storms. Generally the farther southeast your location the higher
the rain chances, but the question is how far north some of this
activity may extend. A couple runs of the HRRR have been quite
aggressive in terms of convective coverage, and as a result, it
has been consolidating cold pools and sending an outflow boundary
as far north as I-20 which would cause showers/storms to develop
as far northwest as the DFW Metroplex. This solution is the
outlier by far, and have only brought PoPs as far northwest as the
Hillsboro/Ennis area for now. Any stronger storms that do manage
to develop will maintain a downburst wind threat, but a hail
threat seems low given the unfavorable shear.

In the meantime, a massive upper trough is beginning to deepen
across the western half of the country. Increased troughing to
our west will cause some of the features that have brought storms
to our western counties the past couple of days to pull back
northwestward, including the stalled frontal zone in the High
Plains and diffuse dryline in West TX. This also includes the low-
level thermal ridge which was positioned in western North TX
yesterday afternoon, delivering some 100+ degree temperatures to
our western counties. With 850mb temperatures decreasing a few
degrees from yesterday, surface temperatures will follow suit.
These slightly cooler temperatures will also help hold heat index
values below 105 this afternoon. With the dryline and little
remaining ascent now positioned northwest of the forecast area,
have confidently removed PoPs from our northwestern counties
today, with any activity expected to remain in western OK and the
TX Panhandle.



.LONG TERM... /Issued 325 AM CDT Thu Sep 21 2017/
/Friday through Thursday/

No significant changes have been made through the middle part of
next week with above normal temperatures through the weekend and
then a frontal system expected next week. Through the weekend, an
upper level ridge centered over the Midwest will extend into North
and Central Texas while an upper level trough persists over the
western half of the country. Warm and mostly dry conditions are
expected Friday through Sunday, but a few opportunities for
isolated showers or perhaps a thunderstorm may exist across our
southern and eastern counties.

Model guidance is in good agreement that isolated to scattered
showers and storms will move north from the Gulf Coast on Friday,
and more isolated afternoon convection may develop across parts of
East Texas. Majority of this activity should remain outside of our
CWA, but it`s possible one or two showers could encroach into our
southern or eastern counties. On Saturday, drier theta-e air
moving across the region should squash most convective potential.
On Sunday, a weak disturbance retrograding across the southern
periphery of the upper level ridge will weaken the influence of
the ridge and may transport some better moisture into the region.
While the forecast remains dry for now, low chances for rain may
need to be introduced by the end of the weekend. Temperatures
Friday through Sunday will remain largely in the lower to mid 90s
with overnight lows in the upper 60s to mid 70s.

Beginning Monday, the influence of the upper level ridge will be
replaced by southwest flow aloft as the western CONUS trough
starts to edge east. However, both operational and ensemble runs
keep the main axis of the trough positioned over the western
CONUS until later in the week. By Thursday/Friday, a piece of
energy appears to disconnect from the trough over the Desert
Southwest while the northern portions of the trough race east in
response to a strong shortwave moving through central and eastern
Canada. A cold front will move down the Plains early in the week,
but stall to our northwest and north on Tuesday. With the help of
disturbances moving through the southwest flow aloft and southerly
winds in the lower levels of the atmosphere maintaining a
constant stream of moisture into the region, rain chances will
increase across the region. The highest rain chances will first be
across our western and northwestern counties (closer to the
location of the front). Convection along the front may help to
slide the front south sooner, but by Wednesday, the front may
start sinking south as the northern portions of the upper level
trough move across the northern Plains. There is some indication
that the slow moving nature of this system may result in a heavy
rain threat for part of the region, but there are still many
details to be refined about this system. The coolest and driest
air behind the front is currently delayed until late in the week
when a shortwave drops southeast through the Plains. Temperatures
next week will be cooler at first due to the increased cloud cover
and absence of the upper level ridge, and then due to the rain
and front.



Dallas-Ft. Worth    96  76  93  75  93 /  10  10   5   5   5
Waco                95  75  92  72  93 /  20  10  10   5   5
Paris               92  71  91  71  91 /  20   5  10   5  10
Denton              95  73  93  72  92 /  10  10   5   5   5
McKinney            93  73  92  72  91 /  10   5   5   5   5
Dallas              96  77  94  75  94 /  10   5   5   5   5
Terrell             93  73  94  72  93 /  20   5   5   5   5
Corsicana           94  73  92  72  93 /  20  10  10   5   5
Temple              93  73  92  71  92 /  20  10  10   5   5
Mineral Wells       97  72  93  71  92 /  10  10   5   5   5




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